This manual has been produced to familiarize data users with the design, and the procedures followed for data collection and processing, in the base year and first follow-up of the High School Longitudinal Study of 2009 (HSLS:09), with emphasis on the first follow-up. It also provides the necessary documentation for use of the public-use data files and information that will be helpful to analysts in accessing and understanding the restricted-use files. Chapter 1 serves as an introduction to HSLS:09. It includes an overview and history of the National Center for Education Statistics (NCES) program of longitudinal high school cohort studies, summarizes the HSLS:09 objectives, and supplies an overview of the base-year and longitudinal study design. Chapter 2 describes the base-year data collection instruments, including both the development and content of questionnaires (in the first follow-up, the student, parent, counselor, and school administrator questionnaires). Chapter 2 also provides information on the development of the mathematics assessment in algebraic reasoning and the scoring procedures and psychometric characteristics of this assessment. The sample design used in the base year and first follow-up is documented in chapter 3. Data collection methods and results--including schedules, training, procedures, and response rates--are presented in chapter 4. Chapter 5 describes data preparation and processing, including the receipt control system, coding operations, machine editing, and data file preparation. Additionally, chapter 5 provides information on the data preparation, scaling, and psychometric characteristics of some of the scales used in the first follow-up student and administrator surveys. Chapter 6 describes weighting, variance estimation, and unit nonresponse bias estimation, while chapter 7 examines item-level statistical issues such as item nonresponse bias, imputation, and disclosure risk analysis. Chapter 8 describes the contents of the data files, including the data structure and linkages to other databases. [For the appendices to this report, "High School Longitudinal Study of 2009 (HSLS:09) Base Year to First Follow-up Data File Documentation. Appendixes. NCES 2014-361," see ED565693.]